They are torqued on pretty tight and should be put back on tight. They were made like this to simplify the manufacturing process, not to facilitate cleaning. If it was ment to never be removed they would have pinned it or silver soldered it. Good ole common sense should be considered when handling any of these cannons, and I agree it should never be fired if the cascabel is loose. 
Not correct! Breech plugs like Gun barrels in more modern guns are installed and not intended to be removed-except perhaps for repairs.
During the muzzle loading gun era they rarely if ever pinned and never soldered in place. Some old guns have ben found with what appears to be asphaltum on the threads. Some say this is not asphaltum, rather a residue of black powder fouling and oil the has seeped by the breech seal into the threads.
Breech plugs were installed as a matter of necessity as a way close the bore. When turned in they should have a moderate
interference crush fit which seats the face of the plug and slightly stretches and springs the threads for a lock. Casual and periodic removal will eventually stretch the threads enough that they will no longer stretch and lock.
This was something that was emphasized back in College in black powder gun making, that breech plugs should not be remove unless absolutely necessary. If they were then also consider fitting a new plug.
It is correct these imported cannons have screw in breech plugs as a simplification of the manufacturing process. These things are made by cutting off a section from a long rifled blank, that is then profiled, tapped and the breech plug screwed in. If it was intend the breech plug be periodically removed they would have provision for use of a spanner or wrench-they do not.
